1. Field of the Invention
The present invention relates to a method of fabrication of a control transistor for an electrooptical flat-panel display screen and to a control transistor fabricated in accordance with said method. The invention is primarily applicable to the construction of large-area liquid crystal screens and more particularly to integration of control elements of the screen in the form of thin films.
2. Description of the Prior Art
As is already known, screens of this type usually have a large number of picture elements (pixels) of square or rectangular shape. These picture elements have to be addressed individually. The screen definition is a function of the number of elements which are capable of receiving an item of information. Control of each element is performed by applying an electric field through the liquid crystal. In order to meet video data display requirements, matrix-type displays have been proposed. In a display of this type, each picture element or so-called pixel is defined by the intersection of two orthogonol arrays of row and column leads.
Addressing of a pixel by means of control voltages applied to the row and column concerned does not need to be maintained if a time-multiplexing technique is adopted for refreshing the state of the screen by recurrence. This technique is based on a persistence effect which may be either physiological or available within the screen element.
Since the switching elements are addressed sequentially, one row after another, the number of rows which can thus be addressed is usually limited by the characteristics of the electrooptical effect of the liquid crystal employed. It is possible to address a large number of rows (&gt;100) only at the expense of the other characteristics of the screen (reduction in contrast and increase in angular dependence). In order to improve the performance of these screens, a transistor or a nonlinear element can be placed in series with each pixel (which constitutes a capacitor). In such a case, the array behaves like a memory cell.
The present invention relates to a screen controlled by a matrix array of transistors and to a method for the construction of said screen.
In the field of display screens, current technical requirements are primarily centered on the achievement of higher image definition. In the case of screens of the matrix display type, there is consequently a design trend towards display devices having a large number of addressing rows or columns which can amount to as many as 512 or even 1024. This entails a corresponding increase in the number of control transistors. For the purposes of large-scale production, it is necessary in particular to obtain a high production yield, good reproducibility and high stability of these components. A further requirement is that the electrical capacitance of the component must be matched with that of the associated cell, also with good reproducibility.
The construction of a matrix array of thin-film transistors (TFT) usually calls for at least four photoetching steps (photolithographic technique) with high standards of positioning accuracy. These transistors are usually designed in a reverse-biased multistage structure.
In another known design, however, the transistor matrix array has a forward-biased multistage structure as described in the article entitled "Large LCD panel addressed by 320.times.320 TFT array" by J. Richard et al. published in Eurodisplay 84, Paris (1984). The technique described in this article makes it possible to limit the fabrication process to two photoetching steps but is subject to drawbacks such as columns of ITO which has higher resistivity than the metal and difficult etching of ITO which is in a totally oxidized state.
Some of these disadvantages have been solved by increasing the number of photoetching steps of five as described in the article entitled "A 31-inch a-Si TFT addressed color LCD" by Toshio Yanagisawa et al. published in Eurodisplay 84, Paris (1984).
The invention relates to a method which, while retaining the advantage of fabrication of transistors by means of two photoetching steps which do not entail the need for critical positioning, makes it possible to remove the disadvantages mentioned above and to suppress production constraints by adding only a third photoetching step for the fabrication of the screen as a whole.